1. Field of the Invention
The present invention relates to the manufacturing of light-emitting diodes. More specifically, the present invention relates to a technique for fabricating indium gallium aluminum nitride (InGaAlN) semiconductor material on a silicon substrate and transferring such material onto a metal substrate.
2. Related Art
Light-emitting diodes (LEDs) fabricated with InGaAlN semiconductor material have been widely used in large screen displays, traffic lights, lighting sources for backlighting, solid state lighting devices, and so on. Typically, an InGaAlN multilayer structure is epitaxially fabricated on a sapphire or SiC substrate, and electrodes are often placed on the same side of the structure.
A number of methods have been developed to manufacture LEDs. However, InGaAlN LEDs fabricated with these methods typically have low luminance efficiency and low thermal conductivity. This is because a translucent conductive layer can absorb light emitted from the active region. In addition, the same-side electrode configuration often leads to low utilization of a device's surface area.
Alternatively, the device can have a vertical-electrode configuration, wherein one electrode is placed above the epitaxial structure and another electrode is placed on the backside of a conductive substrate (such as a Si substrate). However, with the vertical-electrode configuration, the light absorption problem in the non-translucent substrate and the ohmic contact layer remains. Furthermore, the vertical-electrode configuration can increase the turn-on voltage of the LEDs because of the presence of an aluminum nitride (AlN) buffer layer.
One method for fabricating InGaAlN LEDs employs wafer bonding in conjunction with wet etching techniques. In this method, an InGaAlN multilayer structure is fabricated on a silicon growth substrate. Then the InGaAlN structure and a conductive substrate are pressure-welded at a metal bonding layer. Next, the silicon growth substrate is removed by wet etching, and the InGaAlN structure is thus transferred onto the conductive substrate. This flip-chip fabrication technique allows the device structure to use a vertical-electrode configuration, in which the two electrodes are on both sides of the device. Consequently, the LEDs can have higher luminance efficiency, lower turn-on voltage, and higher surface-area utilization, compared with those fabricated using the other conventional methods. However, during the pressure-welding process, the InGaAlN structure and the conductive substrate are exposed to relatively high pressure and high temperature, which can reduce the reliability of the LEDs. Therefore, the stability of the pressure-welding process is a major concern for this method.
Although sapphire can also be used as a growth-substrate material, a sapphire substrate is more expensive and more difficult to process than a silicon substrate. Therefore, InGaAlN LEDs fabricated on a silicon substrate have higher commercial values.